Heretofore, for example, Patent Literature 1 below proposes an optical combiner to combine light beams emitted from a plurality of laser light sources with one optical fiber.
A multiport coupler described in Patent Literature 1 includes a signal fiber 5 disposed in the center and a plurality of pumping fibers 4 integrally disposed around the signal fiber 5, in which the diameter of the tip end of the multiport coupler is decreased. Around a core 6 of the signal fiber 5 disposed in the center, a radiation confinement waveguide portion 7 is concentrically provided. The radiation confinement waveguide portion 7 has an outer diameter greater than the outer diameter of the core 6, and has a refractive index greater than the refractive index of a cladding 8 and smaller than the refractive index of the core 6.
The radiation confinement waveguide portion 7 confines returning light leaking from the core of a cladding pump fiber 3 at the connecting portion of the multiport coupler to the cladding pump fiber 3 connected at the end portion of the multiport coupler where the diameter is decreased. Thus, the radiation confinement waveguide portion 7 reduces the failure of a pump light source caused by the returning light.
[Patent Literature 1] Japanese Patent No. 5089950